Abstract
This study investigated the impact of different light spectra on growth characteristics and carotenoid accumulation-particularly lutein-in African marigold (Tagetes erecta L.) under controlled conditions. A split-plot design was employed, with four light treatments (monochromatic red, blue, and white, and a combined red-blue light at a 3:1 ratio) as the main factor and two cultivars (Antigua Orange and Antigua Yellow) as the sub-factor. Key morphological traits, photosynthetic pigments, and carotenoid levels were measured to identify the optimal light conditions among the four tested treatments for growth and specialized metabolite enhancement. Red light significantly increased root biomass, root volume, and leaf area, highlighting its positive effect on root and vegetative development. In contrast, blue light elevated the chlorophyll fluorescence index, suggesting improved photosynthetic efficiency, although it was less effective in promoting biomass accumulation. The combined red-blue light treatment promoted balanced growth of both vegetative and reproductive traits, along with the highest total chlorophyll and carotenoid content. White light, with its full-spectrum, delayed flowering but produced the largest flower diameter and the highest flower biomass. Carotenoid profiling revealed a marked increase in lutein content under red light, underscoring its potential to enhance the nutritional and pharmaceutical value of T. erecta. These findings emphasize the importance of customized light spectra in controlled-environment agriculture for optimizing growth, yield, and the production of high-value specialized metabolites. The results offer a valuable foundation for future research on light-mediated modulation of plant physiology and bioactive compound synthesis.